1. Field
This invention is in the general field of hydrated glasses. Specifically, the invention is concerned with optical glass articles which can be prepared from such glasses.
2. Prior Art
Glass articles useful for fine optical applications commonly require a high degree of surface smoothness so that the transmission or reflection of light will not be adversely affected by surface roughness. Typically, to assure surface smoothness of precision optical quality, the surface roughness of the glass should not exceed one-tenth of the wavelength of light being transmitted. Since the wavelengths of visible light range from about 16 microinches (16.mu. inches) to 30 microinches (30.mu. inches), this means that the surface roughness should not exceed about 1.6 microinches in the case of violet light and 3.0.mu. inches in the case of red light.
The relative surface smoothness (or low degrees of surface roughness) of an article can be precisely measured by known means. For example, in one method, the surface characteristics of an article are measured with an instrument which amplifies and displays the displacement of a stylus-like arm which is slowly drawn across the surface of the article. The results can be viewed on a chart as an actual reproduction of the surface being examined, amplified as high as 100,000 times. Alternatively, the results can be described in terms of "Roughness Height". This expression, for purposes of defining a standard, is described as the arithmetical average (AA) deviation expressed in microinches (.mu. inches) measured normal to the centerline. Arithmetic Average (AA) is also known in British Standards as Center Line Average (CLA). This terminology is explained more fully in the publication, Surface Texture, ASA B 46.1-1962, published by the American Society of Mechanical Engineers, New York, New York. Typical of the instruments which can measure fine degrees of surface smoothness (peaks and valleys) are instruments known commerically as a Proficorder or a Surfanalyzer.RTM.. Detailed descriptions concerning the use of such instruments can be found in manuals used with the instruments and other publications. According to one such manual which accompanies the Gould Surfanalyzer.RTM., Model 1200, the results of surface measurement, expressed in AA, can be converted approximately to the root mean square (rms) average by multiplying the AA by 1.11. Hence, surface smoothness, or a low degree of surface roughness, can be expressed in AA units or rms units over a given surface. An example of rms measurements is described in an article entitled "Polishing of Supersmooth Metal Mirrors", Applied Optics, Vol. 14, No. 8, pp. 1808-1812, August, 1975. See also an article entitled "Surface Characterization: A Total Approach", Research/Development, November, 1975.
As used herein, the expression "optical quality surface" or its equivalent, refers to a glass surface having a "Roughness Height" the AA of which is less than 3.0 microinches (3.0.mu. inches) over a surface distance of at least 0.1 inch. A preferred optical quality surface has a Roughness Height the AA of which is less than 1.6 microinches (1.6.mu. inches) over a surface distance of at least 0.1 inch, so that the entire range (16 to 30.mu. inches) of visible light can pass on or through the surface with minimal diffusion or scattering.
A glass article having an optical quality surface is prepared conventionally in three basic steps. Firstly, a glass blank consisting of given glass forming ingredients is formed by conventional means. Secondly, especially in applications where a surface curvature is required (e.g. spherical or aspherical, convex or concave), it is commonly necessary to subject the glass blank to a relatively rough grinding step to develop the approximately desired plane or curvature. Thirdly, there is a fine polishing step which transforms the surface having the approximate plane or curvature to a fine surface of optical quality having a very precisely controlled surface smoothness. It can be appreciated that the grinding and polishing of glass surfaces for optical purposes require a relatively high degree of skill and/or special equipment, especially for the polishing step where a very high degree of surface smoothness is required.
It should be noted that a surface smoothness of optical quality is often required for a variety of glass articles such as optical lenses and mirrors and that such articles, aside from requiring a high degree of surface smoothness, often require surface curvatures that are difficult to obtain (e.g. aspherical, parabolic surfaces). Such requirements contribute significantly to the cost of preparing high quality articles such as lenses or mirrors. Hence, it would be highly desirable if the time, costs, and equipment requirements associated with the preparation of optical quality surfaces of glass articles could be reduced.
Primarily because of their thermal and rheological physical properties, glass materials conventionally used to prepare fine optical articles cannot be molded by known molding techniques to achieve a surface smoothness of optical quality. In recent years, however, it has been found that certain glass compositions can be successfully hydrated to impart to the glass a relatively low viscosity at moderate temperatures. Such glasses have become known as hydrated glasses because they include varying amounts of water within the glass. See, for example, U.S. Pat. No. 3,498,802 and U.S. Pat. No. 3,498,803, which disclose methods of including water within certain types of glass materials to impart to those materials properties not commonly associated with glass per se. More recently, in U.S. Pat. No. 3,912,481 issued on Oct. 14, 1975 in the names of R. F. Bartholomew et al., entitled "Hydrosilicate Thermoplastic Materials", it has been disclosed that glass articles can be thermoplastically formed at relatively low temperatures by forming such glasses in a two-step process to control the water content of certain hydrated glasses. In the first step, excess water is introduced into a base anhydrous glass. Then, the glass is dehydrated to reduce the water content to a defined range.
Although the above cited patent application does disclose how to obtain some degree of thermoplasticity in certain hydrated glass materials, it has now been found that by critically controlling the water content as well as compositions of hydrated glasses and by applying a defined molding technique, it is possible to prepare glass articles having an optical quality surface and that such a surface can be obtained without subjecting the article to costly grinding and polishing steps. The water content may be controlled via a one-step hydration process, a two-step hydration-dehydration process, or a full hydration process followed by a controlled dehydration by heating in a vacuum. Methods of preparing the articles are described in detail below.